Correlating microstructural features with corrosion resistance

Corrosion behaviour of materials is strongly affected by microstructure. Presence of different phases, secondary precipitates, elemental distribution in the material (in phases, in precipitates and due to segregation/depletion of certain elements around precipitates or along flow lines/slip bands) are the microstructural variables that are directly linked to corrosion degradation in stainless steels and nickel based alloys. The grain boundary energy (or the structure at grain boundary) is also known to have a strong influence on the resistance to sensitization, susceptibility to intergranular corrosion and to stress corrosion cracking. Grain boundary engineering has in fact been used to develop stainless steels and nickel based alloys that are highly resistant to intergranular corrosion and to stress corrosion cracking. Similarly, a high fraction of special grain boundaries in materials is expected to make it more resistant to radiation induced segregation (and to irradiation assisted stress corrosion cracking). The susceptibility of the stainless steel to corrosion is different in the cross-sectional and the longitudinal surfaces due to the morphology of inclusions/segregation regions of elements in the material. This is the basis of the 'End Grain Corrosion' that is observed for stainless steels in highly oxidizing environments. Certain crystallographic directions are known to be more prone to pitting corrosion. Even the crevice corrosion leading to intergranular corrosion in nickel based alloys is shown to be highly sensitive to the nature of the grain boundaries and the twin boundaries resist this attack better than the random boundaries. A number of examples will be provided which illustrate the effect of microstructural features on the susceptibility to various forms of corrosion. (author)